The present invention relates to molded case circuit breakers, and more particularly relates to molded case circuit breakers comprising main electrical contacts in the form of vacuum switch assemblies which provide arcless operation.
Molded case circuit breakers are well known in the art. Examples of such circuit breakers are disclosed in U.S. Pat. Nos. 4,503,408, 4,540,961, 4,683,451, 4,697,163, 4,922,220, 4,951,015 and 4,973,928, each of which is incorporated herein by reference. Such circuit breakers are used to protect electrical equipment from damage due to an overcurrent condition.
Conventional molded case circuit breakers include at least one pair of separable main contacts which may be operated either manually by way of a handle disposed on the outside of the case or automatically in response to an overcurrent condition. In the automatic mode of operation, the separable main contacts may be opened by either an operating mechanism or by magnetic repulsion forces generated by a reverse electrical current loop formed between upper and lower contact arms.
The separable main contacts of conventional molded case circuit breakers include a fixed contact and a movable contact secured to the free end of a rotatably mounted contact arm. The operating mechanism includes a spring biased toggle mechanism which rotates the contact arm to open and close the separable contacts. The operating mechanism also includes a cradle member having a latching surface. A latch assembly includes pivotally mounted latch levers adapted to engage the latching surface on the cradle member to latch the operating mechanism to close the separable main contacts. When the latch lever is disengaged from the latching surface on the cradle member, the operating mechanism causes the separable main contacts to be opened. A pivotally mounted trip bar engages the latch lever. In a normal position, the trip bar allows the latch lever to latch the operating mechanism. However, when the trip bar is rotated, the latch lever becomes disengaged from the latching surface on the cradle member to allow the operating mechanism to trip or open the separable main contacts.
Various means have been used to actuate the trip bar to cause the separable main contacts to be tripped or opened. For example, bimetallic elements have been used. These bimetallic elements comprise strips of dissimilar metals which bend in response to heat generated by persistent high levels of electrical current. Such bimetallic elements are serially connected between the load and line side terminals of the circuit breaker assembly such that all of the current that passes through the circuit breaker passes through the bimetallic element. Bimetallic elements are generally used to protect electrical circuitry or electrical loads from an overcurrent condition, generally about 200 to 300 percent of the nominal current rating of the circuit breaker.
Another known means of actuating a trip bar is an electronic trip unit. Such devices include internal current transformers electrically coupled to electronic circuitry. The current transformers only allow a portion of the current flowing through the circuit breaker to flow through the tripping device. Electronic trip units are adjustable and may provide overload protection as well as short circuit protection, generally 1,000 percent or more of the nominal current rating of the circuit breaker.
In other circuit breakers, a magnetic tripping device is provided. This tripping device actuates the operating mechanism in response to relatively high overcurrent conditions. Such magnetic tripping devices are serially coupled between the line and load side terminals of the circuit breaker. The magnetic tripping device includes a coil and a reciprocally mounted plunger assembly. The plunger assembly includes a plunger carried by a carrier having a hammer portion which engages the trip bar when the plunger is attracted downwardly by the coil. The plunger is biased upwardly by an operating spring during normal current conditions defining a magnetic air gap between the plunger and the coil. When the electrical current flowing through the circuit breaker is sufficiently high, magnetic attraction forces are generated between the plunger and the coil to overcome the upward spring force on the plunger. This causes the plunger to be attracted downwardly until the hammer portion of the carrier strikes the trip bar causing it to rotate to allow the operating mechanism to unlatch and trip or open the separable main contacts.
Conventional molded case circuit breakers generate an arc when the main contacts are separated to interrupt a current. In conventional designs, the arc is typically exposed to the ambient environment. However, in many installations, such as mines, granaries, bakeries, textile mills and petro-chemical plants, such exposed arcs are a safety concern and require special enclosures to be built around the circuit breakers.
Higher voltage, metal clad circuit breakers utilize vacuum interrupters in which the arc generated during contact separation occurs in a vacuum chamber where it is more easily extinguished and does not pose an explosion threat. Typically, these large metal clad breakers utilize a stored energy operating mechanism to generate the force needed to close in on the large currents in the circuits in which they are used. Such mechanisms usually include an arrangement of levers driven by a rotating pole shaft to open and close the vacuum interrupter contacts.
A need exists for relatively small and inexpensive molded case circuit breakers which do not produce exposed arcs during operation.
The present invention provides a molded case circuit breaker in which the main contacts are provided inside at least one vacuum switch assembly. The molded case circuit breaker includes an over-the-center toggle mechanism which opens and closes the main electrical contacts located inside the vacuum switch assembly. Such molded case circuit breakers are particularly suitable for low and medium voltage applications, and eliminate the need for secondary enclosures in installations where the presence of exposed arcs is a safety consideration.
An aspect of the invention is to provide a molded case circuit breaker comprising: a housing, at least one vacuum switch mounted in the housing and comprising a vacuum envelope containing a fixed contact assembly and a movable contact assembly movable along a longitudinal axis between a closed circuit position in electrical contact with the fixed contact assembly and an open circuit position spaced from the fixed contact assembly. The molded case circuit breaker further includes a pole arm, a support frame mounting the pole arm for pivoting in a plane of rotation, and an operating mechanism incorporating a toggle mechanism which rotates the pole arm. A mount supports a driven member for movement along a path substantially parallel to the longitudinal axis of the moving contact assembly. A drive link connects the driven member to the pole arm to translate rotation of the pole arm into linear movement of the driven member, and a coupler couples the driven member to the movable contact assembly to move the movable contact assembly between the open and closed positions with the linear movement of the driven member.
This and other aspects of the present invention will be more apparent from the following description.